Desalination through methane hydrate

ABSTRACT

In one embodiment, this invention pertains to desalination of seawater by eding methane into seawater at a depth exceeding 100 meters to form methane hydrate which rises to where it is decomposed into methane and water, and recovering water. Methane is recycled.

BACKGROUND OF INVENTION

1. Field of Invention

This invention pertains to desalination of seawater by formation ofmethane hydrate at appropriate temperature and pressure conditions andsubsequent formation of potable water therefrom.

2. Description of Prior Art

Original methods proposed for desalinating seawater involveddistillation where seawater is heated to the boiling point and watervapor is then condensed to form fresh water. Distillation includes theuse of sunlight to evaporate water and then collecting the condensate toform fresh or potable water.

Desalination by distillation was followed by the use of reverse osmosiswhich involves diffusion of fresh water from seawater through asemipermeable membrane due to the high pressure applied to the seawaterfeed tank. Desalination by reverse osmosis is considered more expensivethan desalination by distillation primarily due to the cost of thesemipermeable membranes and the high pressure pumps required.

Presently, desalination of seawater is effected by freezing. In indirectfreezing, freezing is accomplished by circulating a cold refrigerantthrough a heat exchanger to remove heat from the seawater. Ice is formedon the heat exchanger surface and is removed, washed and melted toproduce fresh water. In the category of freeze desalination by directfreezing, where desalination is carried out by the vacuum freezing vaporcompression process, heat is removed from seawater by direct contactwith a refrigerant. In a secondary refrigerant mode of this process, arefrigerant that has low solubility in water is compressed, cooled to atemperature close to the freezing temperature of salt water and mixedwith seawater. As the refrigerant evaporates, heat is absorbed from themixture and water freezes into ice.

Various alternative proposals for freezing desalination are described inpaper entitled "Desalination by Freezing" by Herbert Wiegandt, School ofChemical Engineering, Cornell University, March 1990.

In gas hydrate or clathrate freeze desalination, a gas hydratespontaneously is formed of an aggregation of water molecules around ahydrocarbon at temperatures higher than the freezing temperature ofwater. When gas hydrate is melted, fresh water and the hydrocarbon arerecovered, thus simultaneously, producing fresh water and thehydrocarbon which can be recirculated. This has the advantage over otherdirect freezing processes in that the operating temperature is higher,thus reducing power requirements when forming and when melting the gashydrates.

U.S. Pat. No. 5,553,456 to McCormack, discloses a clathrate freezingdesalination system and method in which a clathrate forming agent isinjected through a submerged pipeline to a predetermined ocean depth atwhich the surrounding ocean temperature is less than the clathrateforming temperature. The agent combines with the salt water to form aslurry of clathrate ice crystals and brine. The pipeline is concentricand coaxial with a surrounding outer pipeline in which the slurry isformed. The slurry is pumped back to the surface through the outerpipeline and the ice crystals are washed to remove brine. The washedcrystals are then melted and the resultant water is seperated from theclathrate forming agent, which may be discarded or recycled forre-injection through the inner pipeline. The melting of the clathrateice as well as the cold water and air circulating in the desalinationplant can be utilized as a source of air conditioning for localbuildings and facilities.

The clathrate forming agents disclosed by the U.S. Pat. No. 5,553,456include carbon dioxide, halogenated methanes and ethanes, andcyclopropane. A clathrate is a generic term for an inclusion compoundcomposed of water and other molecules of smaller size. Methane hydrateis a specific clathrate.

OBJECTS AND SUMMARY OF INVENTION

It is an object of this invention to desalinate seawater by the use ofmethane hydrate.

It is another object of this invention to form methane hydrate inseawater instantaneously in the hydrate stability zone.

It is another object of this invention to purify polluted water.

These and other objects of this invention are accomplished by a methodof desalinating seawater by feeding methane gas into a lower zone of abody of saline or polluted water of sufficient temperature and pressureto spontaneously form methane hydrate which rises under its own buyoncyafter its formation to a higher zone where it decomposes into methaneand desalinated water, and recovering the desalinated water. Recoveredmethane is recycled to the deep end of the pipeline.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1(a) is a schematic illustration of apparatus for making methanehydrate form seawater.

FIG. 1(b) is a schematic illustration of gas-water separator.

FIG. 2 is a phase diagram of water-ice-methane hydrate stability zonefor fresh water.

DESCRIPTION OF PREFERRED EMBODIMENTS

This process can be used to separate water from heavier or lighterpollutants relative to water. In the case of heavier pollutants, such asbrine or salt, described as desalination, water collects at top of acolumn above brine since water is lighter than brine. In the case ofseparating water from lighter pollutants, such as oil, such pollutantscollect above water and are selectively removed at a point above water.

The desalination process of this invention is now described by referenceto the diagram of the apparatus shown in FIG. 1(a). Column 20 is closedat top 22, open at bottom 34 and is defined by wall 26 on the outerperiphery. Column 20 is typically a pipe that is closed at the top andopen at the bottom. The column is typically positioned vertically in abody of seawater deep enough so that the seawater at the lower portionof the column is sufficiently cold and under sufficient pressure to formmethane hydrates. Although it is possible to artificially controltemperature and pressure, however, sufficient temperature and pressuretypically prevail in a normal ocean profile to naturally form methanehydrates at ocean depths.

FIG. 2 is a phase diagram which defines coexistence of methane gas, ice,water and methane hydrates. FIG. 2 indicates region one of methane gasand ice separated from region two of methane gas and seawater by avertical ice-seawater phase boundary 100. Region three of methanehydrate, ice and methane gas is separated from region four of methanehydrate seawater and methane gas by a vertical ice-seawater phaseboundary 100. Regions one and two are separated from regions three andfour by the hydrate-methane gas phase boundary 200 which extends fromabout 100-meter depth to a depth of about 10,000 meters and below. Itshould be understood that methane hydrate can exist and can be formed atdepths exceeding 10,000 meters. The phase diagram of FIG. 2 indicatesthat methane hydrate can remain stable at depths in the ocean exceedingabout 100 meters and at temperatures below about 30° C.

Pursuant to the phase diagram of FIG. 2, one end of column 20 is locatedwithin region three or four. Since region four has in equilibriummethane hydrate, seawater and methane gas, the column should be locatedin region four or at a depth where temperature is above 0° C. so thatthere is no need to supply energy to melt ice. Naturalpressure-temperature variation in the ocean allows methane pumped intoseawater at depth to spontaneously form methane hydrate.

When the column is located in the sea, temperature of the seawater willbe conducive to formation of methane hydrate as a natural function ofpressure increase and temperature decrease with depth. If the watertemperature is too high, the column must be disposed in regions one ortwo where methane hydrate is not stable and its formation is notnaturally effected. If the water temperature is too low, methane hydrateformation can be effected but at a depth that may not be practical.

The column is typically located at about the boundary line of methanehydrate-methane gas so that the lower portion of the column is below theboundary line and the upper portion of the column is disposed above theboundary line. Such disposition of the column is done so that formationof methane hydrates is facilitated in the lower portion of the columnand decomposition thereof is facilitated in the upper portion of thecolumn. The method includes the step of feeding or injecting methaneinto column 20 through line 28 at point 30 where methane enters thecolumn and instantaneously forms methane hydrate and bubbles filled withmethane. Since mole ratio of methane gas to fresh water is assumed to be1:6, based on the approximate mole ratio of methane gas to water in amethane hydrate, amount of methane fed into the column should be about1/6 of recovered fresh water, on a mole basis. Size of the bubblesvaries and it can be controlled by known means. Smaller bubbles are moredesirable because they facilitate methane hydrate formation.

Methane hydrates and the bubbles are formed in the lower section 32 ofcolumn 20, which section 32 is also referred to herein as hydratestability zone. A plurality of methane hydrates form on the periphery ofthe bubbles, and as the bubbles rise in the column due to theirbuoyancy, so do methane hydrates. Solid methane hydrate, like water ice,is naturally buoyant. Because the unhydrated gas, or the gas in thebubbles, expands with ascent, it will break the bubble shell and a newbubble shell or methane hydrate will form. This process of naturalrupture and continued crystallization converts virtually all methane inthe bubbles to methane hydrate.

Lower section 32 of column 20 is disposed above inlet point 30 formethane but below the phase boundary 36 so that formation of methanehydrates is facilitated. As the methane hydrates form, heat of fusion isgiven off and the heat is absorbed by surroundings giving thesurrounding medium a tendency to rise. However, the insignificant amountof heat given off and the size of the column relative to the body ofwater that it is in make the impact of the tendency negligible. Thelower end of the column in the hydrate stability zone may incorporatevarious fins to facilitate heat exchange to the cold seawatersurrounding the column.

The methane hydrates are formed in section 32 of column 20 and mayattach themselves to the bubbles which are also formed in section 32.Section 32 can be many meters long and diameter of column 20 must belarge enough for the bubbles and methane hydrate to rise unobstructed byice. Also, section 32 must be below the hydrate-gas phase boundary line200 so that temperature and pressure of the seawater in section 32 isconducive to formation of methane hydrates. Seawater in section 32 ischaracterized by natural low temperature and high pressure.

Section 32 also has the effect of natural fractionation which is inevidence wherever heat transfer takes place. As methane hydrates andbubbles containing methane are formed in section 32 and heat of fusionis given off, natural fractionation on a small scale takes place wherebythe heavier brine, formed when water is stripped from seawater, sinksafter cooling through the bottom open end 34 of column 20 to mix withthe body of seawater outside of the column. If desired, brine can bewithdrawn from column 20 through line 35 located at the lower portion ofthe column above the open end 34.

As methane hydrates form as a shell on the bubbles, consuming gas, thegas and the hydrates rise. Gas is buoyant, as is methane hydrate, whichhas a density of about 0.9 g/cm³ whereas density of seawater is about1.1 g/cm³. As methane hydrate and unhydrated bubbles pass from section32 above boundary 36, they enter a zone where pressure is lower andmethane hydrates clinging to the bubbles are no longer stable becausethey are in either region one or two above the boundary line 200. It isintended as part of an industrial process to convert virtually allmethane gas in the bubbles to methane hydrate before it reaches thesurface zone of warmer water where it will melt. Once methane hydratespass above boundary 36, they enter section 38 which is at the upperportion of column 20. Since methane hydrates are unstable in section 38due to the section disposition above the boundary line 200, depicted onthe phase diagram, methane hydrate unit cells start to melt releasingwater and methane gas. Since water (fresh or potable water) is lighterthan seawater, it concentrates in the upper portion of column 20, suchas in section 38 and above, and can be withdrawn through line 40disposed above section 38. Decomposition or melting of methane hydrateunit cells is also accompanied by release of methane gas, which can bewithdrawn although line 42 located at top of column 20 by passagethrough column top 22. Recovered methane is typically recycled to thecolumn.

It should be understood that formation of bubbles is not a requisite ofthis separation process. Relative density of methane hydrate andseawater is such that methane hydrate is propelled upward within thecolumn where it is formed.

If bubbles are formed, methane gas within the bubbles progressivelydissipates as the bubbles move upward through the column and the bubblesdiminish in size and number as methane gas is lost upon formation ofmethane hydrate. It is desired to make bubbles as small as possiblesince smaller bubbles have a more favorable surface area to volumeratio, and this can be done by passing methane gas through a section offrit in the bottom portion of the column. Frit has the ability to reducethe size of the bubbles by subdividing larger bubbles into smallerbubbles.

Methane hydrate can be decomposed in the colum by absorption of heatfrom the surrounding warm surface water to form hydrate slush.Alternatively, the hydrate slush can be sprayed with fresh water, theheat in which is sufficient to decompose the methane hydrate. In anycase, fresh water and methane gas can be separately recovered becausethe solubility of methane in water is very low.

The hydrate melt section can also be many meters long and the longer itis, the more time is given to fully decompose the methane hydrates intowater and methane gas using heat from surrounding water. Since a waterhydrate unit cell is composed of 46 host molecules of water and 1-8guest molecules of methane gas, decomposition of a methane hydrate cellyields 46 molecules of water and 1-8 molecules of methane gas. On a molebasis, it is estimated that one volume of water can accommodate 70 toover 160 volumes of methane gas. When decomposing one volume of methanehydrate, amount of water obtained is slightly less than one volume ofmethane hydrate and amount of gas is about 70 to over 160 volumes atSTP.

Decomposition of the methane hydrates is accompanied by absorption ofheat of dissociation which has the tendency to cool surrounding medium.Absorption of heat in section 38 is accompanied by a minor naturalfractionation whereby the cooled medium has a tendency to sink and awarmer medium to take its place.

FIG. 1(b) illustrates gas-water separator 50 which can be provided attop of column 20 to separate fresh water 52 through line 40 from methane54 which can be removed through line 42. Top of column 20 can beconverted to separator 50 of FIG. 1(b) by providing at the top of column20 gas head 56 within which methane 54 accumulates and a side line 40,shown in connection with the apparatus of FIG. 1(a). Feeding of methanegas through a conduit into column 20 is controlled to maintain freshwater level between levels 60 and 62 with line 40 therebetween and gashead 56 above level 60.

Methane hydrate unit cell is composed of a plurality of host watermolecules and at least one guest molecule of methane. The watermolecules form cages or sites within which are disposed methanemolecules, with one methane molecule per cage, although some cages areempty and do not contain a methane molecule. Such materials are known asinclusion compounds. The water molecules and the methane molecules in amethane hydrate are held together by Van der Waal's forces. Due to therelative size of the cage interior and a methane molecule, only oneguest methane molecule can be accommodated in a host water moleculecage.

More specifically, a unit cell of a methane hydrate is composed of 46water molecules and 1 to 8 molecules of methane. Methane hydrate is anonstochiometric crystalline material in that a variable amount of gasup to a maximum allowed by the crystal lattice structure can becontained within the guest unit cell. The number of methane molecules inthe hydrate unit cell increases with lower temperature and higherpressure, which means that more methane molecules will be present inmethane hydrates formed at greater depths in an ocean, assumingsufficient methane to occupy lattice sites. Typically, methane hydratesare undersaturated in that some of the guest sites are not occupied. Afully saturated methane hydrate, however contains more gas and has ahigher energy density, in btu/liter, than liquified methane because themethane sites in the crystal lattice are closer together than methanemolecules in a liquid.

Methane hydrate is a solid, waxy crystalline material and has thefollowing physical properties:

heat of fusion/dissociation 54 kJ/mol at 273K

heat capacity 257 kJ/mol

heat of solution 13.26 kJ/mol

coefficient of expansion 2/7

density 0.9 g/cm³

The desalination process has been described in connection with formationof methane hydrates in seawater. It should be understood that methanehydrates can be formed in any body of water as long as temperature andpressure are such that they define a hydrate stability zone; i.e., zones3 and 4 on the phase diagram of FIG. 2. This process can be used in abody of polluted water to produced purified fresh water since upondecomposition, a methane hydrate releases methane gas and pure water. Sothe term "purification" and derivatives thereof includes desalination.Furthermore, the term "polluted water" includes saline water.

It was already disclosed that methane hydrate forms naturally andinstantaneously at appropriate temperature and pressure. For purposesherein, the term "instantaneous" means a period of time typically lessthan about 5 seconds, more typically less than a couple of seconds, suchas 0.5-2 seconds.

Rate of desalination depends on many parameters, especially size ofcolumn, injection rate of methane, salinity or impurity of water, anddepth at which the column is located. However, for a column pipe 2meters in diameter, 100 meters in length submerged in a body of seawaterat a depth of 500 meters and with a methane feed rate of 12 m³ /min,rate of desalination or production of fresh or potable water is about310,000 m³ per 24 hour day.

One of the products produced by the process described herein is fresh,potable water suitable for drinking by humans. Solubility of methane infresh water is at a ppm level which is not harmful to humans. Presenceof methane molecules in water can be detected at ppm level. This is oneway that the product fresh water can be tagged with this process sinceit is effective with clathrates wherein the hydrate contains othermolecules than methane, assuming at least some solubility of the othermolecules in the water.

While presently preferred embodiments have been shown of the inventiondisclosed herein, persons skilled in this art will readily appreciatethat various additional changes and modifications can be made withoutdeparting from the spirit of the invention as defined and differentiatedby the following claims.

What is claimed is:
 1. A method for purifying polluted water comprisingthe steps of(a) feeding methane gas into a lower zone of a body ofpolluted water of sufficient temperature and pressure to form methanehydrate which rises after its formation to a higher zone where itdecomposes into methane and purified water, and (b) recovering thepurified water.
 2. The method of claim 1 wherein the lower zone islocated below the boundary line defined by a phase diagram whichseparates regions where methane gas coexists with the polluted water orice from regions where methane hydrate coexists with methane gas and thepolluted water or ice.
 3. The method of claim 2 wherein the methanehydrate is formed instantaneously in the lower zone.
 4. The method ofclaim 3 wherein in the lower zone the temperature is about -20° to about+30° C. and pressure therein is about 10 to about 1,000 atmospheres andin the higher zone temperature is above about -20° C. but below itsboiling point and pressure therein is about 0 to about 1,000atmospheres.
 5. The method of claim 3 wherein in the lower zonetemperature is about 0° to about +30° C. and pressure therein is about50 to about 1,000 atmospheres and in the higher zone temperature isabove 0° C. but below its boiling point and pressure therein is about 0to about 1,000 atmospheres.
 6. The method of claim 4 wherein methanehydrate coexists with water and methane gas.
 7. The method of claim 6wherein the lower zone is disposed in the body of polluted water at adepth exceeding 100 meters and down to 10,000 meters.
 8. The method ofclaim 7 wherein a unit cell of the methane hydrate is composed of hostwater molecules and at least one methane guest molecule.
 9. The methodof claim 8 wherein at least some methane hydrate attaches to bubbles ofmethane gas and is carried thereon upwards.
 10. Product made by themethod of claim
 1. 11. A method for desalinating saline water to potablewater comprising the steps of(a) feeding methane gas into a lower zoneof a body of saline water that is at a temperature and under pressuresufficient to form methane hydrate which rises after its formation to ahigher zone where it decomposes into methane and potable water, and (b)recovering the potable water.
 12. The method of claim 11 wherein thelower zone is located below the boundary line defined by a phase diagramwhich separates regions where methane gas coexists with the pollutedwater or ice from regions where methane hydrate coexists with methanegas and the the polluted water or ice.
 13. The method of claim 12wherein the methane hydrate is formed instantaneously in the lower zone.14. The method of claim 13 wherein in the lower zone the temperature isabout -20° to about +30° C. and pressure therein is about 10 to about1,000 atmospheres and in the higher zone the temperature is above about-20° C. but below its boiling point and pressure therein is about 0 toabout 1,000 atmospheres.
 15. The method of claim 13 wherein in the lowerzone the temperature is about 0° to about +30° C. and pressure thereinis about 50 to about 1,000 atmospheres and in the higher zone thetemperature is above about 0° C. but below its boiling point andpressure therein is about 0 to about 1,000 atmospheres.
 16. The methodof claim 15 wherein the lower and the higher zones are disposed in apipe with the lower disposed within the lower portion of the pipe andthe higher zone is disposed within the upper portion of the pipe, andwherein the lower zone is disposed in the body of saline water wheremethane hydrate coexists with water and methane gas.
 17. The method ofclaim 16 wherein the lower zone is disposed in the body of saline waterat a depth exceeding 100 meters and down to 10,000 meters and below. 18.The method of claim 17 wherein at least some methane hydrate attaches tobubbles of methane gas and is carried thereon upwards.
 19. The method ofclaim 18 including the step of recycling methane obtained fromdecomposition of methane hydrate.
 20. Product made by the method ofclaim 11.